The present invention relates to a direct Quadrature Amplitude Modulation (QAM) modulator that uses advanced digital signal processing techniques and feedback control. The invention is particularly suitable for generating 64/256-QAM signals in the 50 MHz-880 MHz range without an Intermediate Frequency (IF) stage and double-conversion, and may be used, e.g., for communicating digital television data via a cable network.
The following acronyms and terms are used:
ASICxe2x80x94Application-Specific Integrated Circuits
D/Axe2x80x94Digital-to-Analog
DSPxe2x80x94Digital Signal Processor
EMxe2x80x94Electro-Magnetic
EMCxe2x80x94Electro-Magnetic Compatibility
EMIxe2x80x94Electro-Magnetic Interference
I/Qxe2x80x94In-phase/Quadrature-phase.
IFxe2x80x94Intermediate Frequency
LOxe2x80x94Local Oscillator
MPSxe2x80x94Modular Processing System
PLLxe2x80x94Phase-Locked Loop
QAMxe2x80x94Quadrature Amplitude Modulation
QPSKxe2x80x94Quadrature Phase-Shift Keying
RFxe2x80x94Radio Frequency
RMSxe2x80x94Root Mean Square
SAWxe2x80x94Surface Acoustic Wave
QAM modulation is commonly used, for example, in many existing cable television network headends, as well as many other possible uses.
FIG. 1 illustrates a conventional double-conversion QAM modulator 100, which includes a modulator portion 102 and an up-converter portion 172. With current systems, a digital modulator 105 in a device termed a MPS generates a QAM signal at an IF, such as a 44 MHz or 36 MHz international standard.
The digital signal output from the modulator 105 passes through a D/A converter 110, a low-pass filter 115, an IF amplifier 120, a SAW filter 125, an IF amplifier 130, and an, IF output driver 140.
An external up-converter 172 receives the IF signal from the IF output driver 140 and translates the IF signal to a RF signal. The up-converter 172 includes a 1.3 GHz mixer 145, a 1.3 GHz band-pass filter 150, a 1.3-2.16 GHz mixer 155, a band-pass filter 160, a RF driver 165 and a 50 to 880 MHz coupler 170 which provides an output signal on line 180. A monitor 175 monitors an output of the coupler 170.
This IF-RF approach requires a two-stage RF converter (the so-called xe2x80x9cdouble-conversionxe2x80x9d technology). Thus, in such current modulators, one digital modulator 105 and a two-stage analog converter are used. That is, the up-converter 172 has two stagesxe2x80x94mixers 145 and 155.
However, the conventional modulation approaches have a number of disadvantages, including:
(1) This technology requires two high frequency local oscillators 145 and 155. There are several disadvantages associated with these local oscillators (mixers) 145 and 155. First, they require more materials, board space and power supply. In addition to cost issues, high frequency signals are a source of EMI and EMC problems. Additional L-band conversion introduces phase noise to the modulated signal. Double-conversion increases manufacturing difficulty and reduces product reliability. In this configuration, the SAW filter hasxe2x80x9420 dB attenuation. More amplifiers are required to compensate the filter loss.
To control the phase noise, each LO 145, 155 has to be phase locked to a reference source using a PLL. Active filters are required for the PLL to provide a good frequency resolution and to reduce RMS phase errors.
(2) The IF output requires the SAW filter 125 to filter out spurious images and harmonics. However, the SAW filter 125 introduces a significant attenuation, such asxe2x80x9420 dB. Accordingly, more amplifiers (e.g., amplifiers 120, 130) are required to compensate for the amplitude loss due to the SAW filter 125. Moreover, these amplifiers 120, 130 have to be adjusted to balance harmonic distortion and signal-to-noise ratio. In the conventional QAM design tested by the inventor, a one-stage amplifier 120 was used to drive the SAW filter 125 and three more stages of amplifiers (in amplifier function 130) on the output to increase the output level to the required level. The second IF amplifier (1.3 GHz) 130 also required multi-stage filtering (in the band-pass filter 150) to remove the image and harmonics. Buffers and amplifiers were also required for these filters.
(3) EMI and EMC are very important issues with high-frequency LOs and mixers. Specifically, very good EM shields are required for all oscillators, mixers and filters. However, these shields not only require circuit board space, but also add difficulties and expense to manufacturing and trouble-shooting.
(4) Double-conversion introduces additional phase noise into the QAM signal.
(5) Double-conversion reduces product reliability.
(6) The cost for modulating each channel is very highxe2x80x94approximately 1,200 xe2x80x94for conventional modulation and upconversion technology.
(7) Separate boxes (e.g., packaging) for the modulator 102 and upconverter 172 make status monitoring, operation control and redundancy control more complicated.
Accordingly, it would be desirable to provide a digital modulation technology that addresses the above problems.
The system should provide improved performance over conventional modulators, and should be implementable in a more compact design and at a lower cost.
The system should provide combined modulation and upconversion in a single package for applications such as digital cable television transmission.
The system should be implementable using off-the-shelf DSP and ASIC devices.
The present invention provides a system having the above and other advantages.
The present invention relates to a direct QAM modulator.
A direct quadrature amplitude modulation (QAM) modulator in accordance with the invention provides amplitude and phase pre-equalization to reduce complexity and cost. In-phase (I) and quadrature-phase (Q) QAM signal components are provided from an analog modulator. A radio-frequency (RF) driver provides a RF signal with the QAM components. A monitoring device monitors phase and/or amplitude errors in the RF signal, and provides a corresponding signal to a digital complex phase and amplitude equalizer embedded in a digital Nyquist filter. The equalizer uses the feedback signal to provide a pre-equalizing signal to a digital-to-analog converter, which provides a corresponding analog equalizing signal. This signal is, in turn, low-pass filtered and fed back to the analog I/Q modulator to equalize the phase and/or and amplitude of the I and Q components there.
The invention is particularly suitable for generating 64/256-QAM signals in the 50 MHz-880 MHz range without an Intermediate Frequency (IF) stage and double-conversion, and may be used, e.g., for communicating digital television data via a cable network.